docs: def _keplerian_circular_to_keplerian(cls, coord, center):
          """Conversion from Keplerian near-circular elements to Mean Keplerian
          """
          a, ex, ey, i, Ω, u = coord
  
          e = sqrt(ex ** 2 + ey ** 2)
          ω = arctan2(ey / e, ex / e)
          ν = u - ω
  
          return np.array([a, e, i, Ω, ω, ν], dtype=float)